The present invention relates to a thin film exhibiting magnetoresistance effects for magnetic sensor such as magnetic head. Ferromagnetic such as permalloy which exhibits magnetoresistance effects have been on the use of various magnetic sensors such as magnetic heads. Magnetoresistance effects are the phenomenon of variation in electric resistance with application of a magnetic field. Such magnetoresistance effect alloy such as permalloy shows an anisotropic magnetoresistance effect of variation in electric resistivity depending upon a relative angle of an orientation of magnetization to a current direction. Permalloy thin films used for magnetic heads of magnetic recorders show an anisotropic magnetoresistance variation of 2-3%.
In recent years, the requirement for high density magnetic recording has been on the increase. For satisfying this requirement, it has been required to develop a material showing a large magnetoresistance variation. A magnetic sensor such as magnetic head is required to detect a slight leakage of magnetic field from a magnetic recording medium, for which reason a material showing a large magnetoresistance variation under a small magnetic field, for example, not more than 100 Oe (Oersteds) (=8 kA/m) is required for highly sensitive magnetic sensor.
In recent years, magnetic multilayers (or superlattices) have been attracted as a material which realizes a giant magnetoresistance effect. In Journal of Applied Physics 67(9), May 1, 1990, pp. 50908-5913, it is disclosed that Fe(001)/Cr(001) superlattices with antiferromagnetic interlayer coupling exhibit a giant magnetoresistance at room temperature when an applied field aligns the magnetization of the Fe layers, the resistivity drops a factor of 2 and this giant magnetoresistance can be ascribed to the spin dependent scattering at interfaces. In Journal of Magnetism and Magnetic Materials 94, 1991, L1-L5, it is disclosed that Co/Cu multilayers with antiferromagnetic interlayer coupling exhibit a giant magnetoresistance at room temperature. Under the zero magnetic field, magnetic moments in the ferromagnetic metal layer (Fe or Co) intervened by the non-magnetic metal layer (Cr or Cu) antiferromagnetically align. With application of magnetic field, the magnetic moments in the ferromagnetic metal layers are changed to ferromagnetically align thereby electrical resistivity is dropped. This phenomenon is called as giant magnetoresistance effect which is different in mechanism from the anisotropic magnetoresistance effects of the permalloy. The above superlattice and multilayer structures have disadvantages in large exchange-interaction between the ferromagnetic layers which causes the orientation of the magnetization of the ferromagnetic layer is unlikely to be changed by an externally applied magnetic field. The magnetic field for saturation of the magnetoresistance is a few kOe to 10 kOe. Such superlattices and multilayers having large saturation magnetic fields are not available for highly sensitive magnetic sensor such as magnetic head.
In order to satisfy the above disadvantages in large saturation magnetic fields with the magnetic superlattices and multilayers, spin valve magnetoresistance effect films have been proposed wherein two magnetic films are structurally and magnetically separated by a non-magnetic spacer layer. This spin valve structure is disclosed in Japanese laid-open patent publication Nos. 2-61572, 4-358310 and 6-60336. In the spin valve structure, first one of the two magnetic films allows the magnetization to be pined whilst second one of the two magnetic films allows the magnetization to rotate in accordance with an externally applied magnetic field. The first one is so called as a pined layer pining the magnetization and the second one is so called as a free layer allowing the magnetization to rotate freely. In the spin valve structure, the magnetoresistance varies depending upon the relative angle of the above pined and free layers. In order to pin the magnetization of one of the magnetic layers, it was proposed that two magnetic metal layers having different coercive forces from each other are provided to sandwich a non-magnetic spacer layer. This structure is disclosed in Applied Physics Letters, 59(2), Jul. 8, 1991, pp. 240-242. Fe--Co--Cu sandwich structure is used, wherein Fe layer is a first magnetic layer having a small coercive force whilst Co layer is a second magnetic layer having a large coercive force and Cu layer is a non-magnetic spacer layer. Alternatively, it was also proposed that two soft magnetic metal layers are provided to sandwich the non-magnetic spacer layer and an antiferromagnetic layer is further provided adjacent to one of the two soft magnetic metal layers so that the magnetization of the soft magnetic metal layer adjacent to the antiferromagnetic layer is pined by an exchange-bias magnetic field due to exchange-interaction from the antiferromagnetic layer whilst another soft magnetic metal layer separated by the non-magnetic spacer layer from the soft magnetic metal layer adjacent to the antiferromagnetic thin film allows magnetization to rotate freely in accordance with the external magnetic field. In Physical Review B, vol. 4, No. 1, Jan. 1, 1991, pp. 1297-1300, it is disclosed that, in spin valve structure, Ni--Fe soft magnetic layers sandwich a Cu non-magnetic spacer layer and a Fe--Mn antiferromagnetic layer is provided adjacent to one of the Ni--Fe soft magnetic layers. Further alternatively, it was also proposed that two soft magnetic metal layers are provided to sandwich the non-magnetic spacer layer and an antiferromagnetic layer having a high electrical resistance and a large coercive force is further provided to contact with opposite ends of one of the two soft magnetic metal layers so that the magnetization of the soft magnetic metal layer in contact with the antiferromagnetic layer is pined whilst another soft magnetic metal layer separated by the non-magnetic spacer layer allows magnetization to rotate freely in accordance with the external magnetic field. In Japanese laid-open patent publication No. 6-325934, two Co--Fe soft magnetic metal layers are provided to sandwich a Cu non-magnetic spacer layer and a Co--Pt--Cr ferromagnetic metal layer having a high coercive force is provided to contact with one of the two Co--Fe soft magnetic metal layers.
The above three type spin valve structures utilize conduction electrons to be scattered in one direction and show a magnetoresistance variation in the range of 5-10%.
In order to obtain a further increase in the magnetoresistance variation, it was proposed that two non-magnetic spacer layers are provided to sandwich a free magnetic metal layer with magnetization to rotate freely in accordance with an externally applied magnetic field. Further two soft magnetic metal layers are provided to sandwich the two non-magnetic spacer layers sandwiching the free magnetic metal layer. Two ferromagnetic layers are provided adjacent to the two soft magnetic metal layers so as to pin the magnetization of the two soft magnetic metal layers whereby a symmetrical dual spin valve structure having five layered structure is formed. This is disclosed in Japanese laid-open patent publication No. 6-223336. The symmetrical dual spin valve structure can utilize conduction electrons to be scattered in any directions. Another dual spin valve is further disclosed in Journal of Applied Physics 78(1), Jul. 1, 1995, pp. 273-277. This symmetrical dual spin valve shows a large magnetoresistance effect exceeding 21%.
The magnetoresistance effect of the above spin valve structure depends upon the relative angle of the magnetic moments between the two soft magnetic layers sandwiching the two non-magnetic metal layers sandwiching the magnetic layer with magnetization free to rotate in accordance with the externally applied magnetic field. The magnetoresistance effect is, however, independent from the current direction. The magnetoresistance effect of the spin valve is thus considered to be generated by an analogous mechanism to that of the above mentioned magnetic superlattices. The magnetoresistance effect of the above spin valve structure differ from the above mentioned magnetic superlattices in providing one or more non-magnetic spacer layers which has such a sufficient thickness as to suppress an interfacial exchange-coupling between the two soft magnetic layers. The spin valve structure is likely to show a relatively large magnetoresistance variation but not exceeding that of the artificial lattice structure such as superlattice structure. The spin valve structure shows an extremely small saturation magnetic field for saturating the magnetoresistance. This means that the spin valve structure is highly sensitive to a slight magnetic field. In Japanese laid-open patent publication No. 6-60336, it is disclosed that the spin valve structure comprises a multi-layered structure of glass/Co(6 nm)/Cu(3.4 nm)/Fe--Mn(10 nm)/Cu(1 nm), which shows a large magnetoresistance variation of 8.7% under a small magnetic field in the range of 20-120 Oersteds. In the above second spin valve structure, the antiferromagnetic layer is used as an exchange-coupled film to pin the magnetization of one of the two magnetic layers whilst the other magnetic layer allows the magnetization to rotate freely in accordance with the externally applied magnetic field. This second type spin valve structure is superior in properties and facilitation of processing the same into a magnetic head, for which reason developments of the second type spin valve structure are more active.
The above antiferromagnetic exchange-coupled layer is the essential layer for the spin valve films. This antiferromagnetic exchange-coupled layer is required both to have a superior corrosion resistance and to provide a sufficiently large exchange-coupling magnetic field to the soft magnetic layer adjacent to the antiferromagnetic exchange-coupled layer. Fe--Mn alloy is capable of providing a large exchange-coupling magnetic field in the range of 200-400 Oersteds for pinning the magnetization of the adjacent soft magnetic layer and also gives a stable magnetoresistance. Nevertheless, Fe--Mn alloy is inferior in corrosion resistance, for which reason if Fe--Mn alloy is applied to the antiferromagnetic exchange-coulped layer included in the spin valve films, it is difficult to ensure a sufficient high reliability of the magnetic head using the spin valve films.
In Japanese laid-open patent publication No. 7-220246, it is disclosed that, in place of the Fe--Mn antiferromagnetic exchange-coupled layer for spin valve films, a NiO film is used as the antiferromagnetic exchange-coupled layer in the spin valve film. Namely, the antiferromagnetic oxide exchange-coupled layer is used for spin valve films. In Japanese laid-open patent publication No. 7-202292, it is also disclosed that, in place of the Fe--Mn antiferromagnetic exchange-coupled layer for spin valve films, NiO/CoO superlattice structure is used as the antiferromagnetic exchange-coupling layer in the spin valve film. Namely, the antiferromagnetic oxide exchange-coulpled layer is used for spin valve films. The antiferromagnetic oxide is superior in corrosion resistance. Such antiferromagnetic oxide exchange-coupled layers are capable of providing small exchange-bias magnetic fields of approximately 100 Oersteds which is insufficient for pinning the magnetization of the soft magnetic layer adjacent to the antiferromagnetic oxide exchange-coupled layer because the maximum leakage magnetic field from the magnetic medium may exceed 100 Oersteds. If the antiferromagnetic oxide exchange-coupled layer is used in the spin valve films for the magnetic head, then the magnetization of the soft magnetic layer adjacent to the antiferromagnetic oxide exchange-coupling layer may rotate in accordance with such a large leakage magnetic field as exceeding 100 Oersteds from the magnetic medium although the magnetization should have to be pinned by the exchange-bias magnetic field provided by the antiferromagnetic oxide exchange-coupled layer. This means that the operations of the magnetic head is unstable.
In the above circumstances, it had been required to develop a novel magnetoresistive spin valve multi-layered structure showing stable performances and properties.